1. Field of the Invention
The present invention relates generally to a failure detection system for an exhaust emission control system for use in an internal combustion engine. More specifically, the present invention relates to an exhaust emission control failure detection system for an internal combustion engine, which is capable of detecting failure or malfunction of the operation of the exhaust emission control system with high accuracy so as to enhance the reliability of the system.
2. Description of the Background Art
In a known exhaust emission control system for an internal combustion engine, a secondary air is introduced into an engine exhaust passage to be mixed with an exhaust gas so as to oxidize unburnt exhaust emission, such as, carbon monoxide (CO) and hydro carbon (HC) by means of a catalytic converter, such as, a catalytic converter rhodium (CCRO) disposed in the exhaust passage. The secondary air is introduced into the exhaust passage in a predetermined engine driving range where an enrichment correction of an air-fuel ratio of an air/fuel mixture is performed, such as, an engine start up enrichment correction, an acceleration enrichment correction and the like. This is because the exhaust gas includes a substantial amount of CO and HC during the enrichment correction of the air/fuel ratio being performed, and thus, the secondary air is supplied to the catalytic converter for oxidation of CO and HC.
Japanese First Patent publication (Tokkaisho) No. 63-165744 discloses a sensor for monitoring degradation of a catalyst used in a honeycomb type catalytic converter. In this publication, the sensor includes an elongate base coated with a catalyst which is the same as that coated on the honeycombed support of the catalytic converter. The sensor is inserted into a hole of the honeycombed support so as to indirectly monitor the degradation of the catalyst used in the catalytic converter.
However, the disclosed sensor is only applicable for monitoring the degradation of the catalytic converter, and thus, it is impossible to detect failure of the other components of the exhaust emission control system, such as, whether the secondary air is properly introduced into the exhaust gas.